a. Field of the Invention
This invention relates to microphones.
b. Background Art
Loudspeakers and microphones essentially comprise a movable diaphragm or other member which provides conversion between a sound pressure wave and an electrical signal.
Microphones are moving from typical analogue microphones to digital microphone modules. These microphone modules typically consist of a sensor manufactured in a micro-electro-mechanical system (MEMS) process and an analogue to digital converter (ADC). The output of the ADC (typically a sigma delta type converter) is a PDM (pulse density modulation) stream that outputs the data to a baseband processor.
This invention is of particular interest for MEMS microphones, which due to their miniaturised size, are particularly prone to signal distortion or damage in the presence of high sound pressure levels. However, the invention is applicable to capacitive microphones generally, such as electret condenser microphones (ECMs).
The acoustical reference levels of 94 dB Sound Pressure Level (“SPL”) results in a typical voltage from a MEMS microphone sensor of 5 mV @ 94 dBSPL. A required signal to noise ratio for the module is typically greater than 60.5 dB. This means 64 dB for the sensor and 64 dB for the ADC.
When there is background noise, particularly wind noise, high sound pressures levels can occur, for example above 140 dBSPL, which is at the limit of the physical capability of the MEMS microphone before damage may occur. This wind noise can also introduce non-linearity to the microphone signal. Removing the noise afterwards in a noise canceller cannot be achieved without distortion, since the signals are distorted and non-linear.
There is therefore a need to provide wind noise rejection. In addition, the demands on signal to noise ratio (“SNR”) are always increasing. It is expected that SNR of greater than 66 dB will be demanded in future, as well as wind noise enhancement.
The limits to improving the SNR derive both from the mechanical design of the pressure sensor and the electric circuit design.
The sensor presents a limitation to the SNR by virtue of the achievable compliance of membrane, and the required small size of the sensor, for example the small distance between the membrane and the back electrode, and the bias voltage to be used. These parameters are of course interdependent and optimised in actual design. The components used in the electronic circuitry determine the SNR which can be achieved in the electrical signal processing.
Wind noise affects the overall audio performance basically in two ways. Firstly, sound evolves very close to the membrane when a turbulent air flow interacts with holes and edges found in the sound inlet path of the microphone. Thus, sound is generated directly at the microphone, which can therefore dominate over the more distant sound being recorded. Secondly, turbulent air affects the operating point of the membrane and therefore introduces non-linearity to the microphone signal.
It is known to use mechanical pop and wind noise shields, but these require extra space which may not be available in some applications. These devices transform the air flow to create as many as possible uncorrelated sound sources by using a fine grid or foam, to spread the flow of energy between multiple sound sources which effectively have different phases over time. This reduces the wind noise impact. The microphone can then remain in the linear operating range, and de-noising algorithms can function properly.
If there is insufficient space for pop or wind noise shields, algorithmic solutions are also known. These involve echo cancellation or noise suppression, mostly in the digital domain, and operate on the one-dimensional microphone signal. These can perform well as long as the transducers operate in the linear range.
However, in the case of non-linearities, introduced by turbulence on the membrane, these algorithms fail to work because of the one-dimensional nature of the microphone signal. In particular, it is difficult to estimate the non-linearities practically latency free.
Wind noise levels can easily reach 140 dB SPL. Measurements on microphones show that non-linear behaviour can be expected for most microphone designs at these levels of sound pressure.